Final answer:
Under perfect competition, firms face the flattest (most elastic) demand curve.
Step-by-step explanation:
A perfectly competitive firm faces the flattest (most elastic) demand curve.
Under perfect competition, the demand curve faced by a firm is perfectly elastic, meaning the firm can sell any quantity it wishes at the prevailing market price.
On the other hand, monopolistic competition, monopoly, and oligopoly face demand curves that are less elastic than perfect competition.